Quadrupole Analyzers, as used in Residual Gas Analyzers (RGAs) and other Mass Spectrometers, require a precisely controlled two phase RF supply signal. The RF supply signal amplitude is of the order of hundreds of volts peak (Vpk) and its frequency is typically a few MHz. The actual voltage amplitude and frequency may depend on the required mass range and various properties of the quadrupole analyzer. A Quadrupole Analyzer application may, for example, utilize a RF supply signal with an amplitude of 187.6 Vpk at 1.8432 MHz, although other voltages and frequencies may be chosen to suit a particular analyzer and mass range.
A Quadrupole Analyzer generally comprises two primary subsystems—the quadrupole subsystem (also referred to herein as the “Vacuum System”), which hosts the quadrupole device and associated hardware, and the signal generation, control, and analysis subsystem, which hosts the RF signal generator, control and analysis components, and other support electronics.
The quadrupole subsystem may be placed in an ionizing radiation environment, which can degrade the semiconductor and other components of the control/analysis subsystem. To protect these vulnerable components, the control/analysis subsystem may be remotely located with respect to the quadrupole subsystem. In such a configuration, the quadrupole subsystem and the control/analysis subsystem may be connected by a cable or other communication medium and separated by a distance that is typically tens of meters.
The RF signal generator is typically a fixed frequency, crystal controlled component with an amplitude modulator configured to precisely control the RF signal amplitude through a control system. The control system comprises an accurate rectifier to measure the RF signal voltage at the quadrupole subsystem, and use the measurement as a feedback signal of the control system to maintain the RF signal amplitude at the required value. The RF signal generator and associated control system is generally a design that is well known in the art.
Common practice is to collocate the rectifier with the quadrupole analyzer subsystem to accurately measure amplitude of the RF signal being applied to the quadrupole. When the quadrupole is used in an ionizing radiation environment, the rectifier may be a thermionic diode type (e.g., vacuum tube), because a semiconductor diode would degrade and/or malfunction in the presence of ionizing radiation. The rectifier output signal must be passed back to the control/analysis subsystem, which is inconvenient and risks the introduction of noise. There is also the concern of obsolescence for the thermionic diode and the complication of implementing its required heater supply.